All-optical switches based on photonic crystal nanocavities

Takasumi Tanabe, Akihiko Shinya, and Masaya Notomi
Optical Science Laboratory

 Small cavities with strong light confinement (high-Q) can be fabricated by employing photonic crystals (PhCs) [1], whose dielectric is periodically modulated. In such small high-Q optical cavities, efficient all-optical modulation based on the light-matter interaction is expected because the photon density inside the cavity becomes extremely high. Silicon-based integrated optical circuits are regarded to have very high potential, due to the possibility of fusion with existing electrical devices. However, there has been some doubt as to whether active all-optical devices can satisfy certain requirements, such as low operation energy, high-speed operation, high switching contrast, and small device size. We will also have to be able to cascade them on-chip. No device meeting these requirements has been fabricated on silicon yet. Here, we demonstrated low operation energy and high-speed all-optical switching on silicon-chip based on a PhC high-Q nanocavity [Fig. 1 (top)].
 The resonant transmission spectrum of a fabricated device under the linear operation condition is shown in Fig. 1 (bottom). When the optical energy of the near-infrared light is converted to thermal energy through the two-photon absorption process, the center of the transmission spectrum of the nanocavity shifts because the refractive index is modulated. By utilizing this phenomena, ultra-low-energy switching operation of a few pJ (pico=10-12) is performed [2]. By utilizing refractive index modulation due to the carrier-plasma effect, further operation energy reduction and switching speed enhancement are expected. As shown on Fig. 2, the all-optical switching operation based on the carrier-plasma effect is demonstrated at few ten fJ (femto=10-15) effective energy with a speed higher than 100 ps [3]. In the case shown in the graph, continues-wave signal light whose wavelength is slightly detuned to shorter wavelength from Mode-S resonance is modulated with pulsed control light, which is Mode-C resonant. This energy value and the device size are the smallest yet reported for fast silicon all-optical active devices, and these are achieved owing to the high-Q and efficient coupling with waveguides that are simultaneously attained in our PhC nanocavities [2].
 The present result opens the way to silicon photonics; namely, the possibility of practical high-speed, low-power, and high-density all-optical logic gate on-chip applications on a silicon PhC platform.
[1] M. Notomi et al., Opt. Express 12 (2004) 1551.
[2] M. Notomi et al., Opt. Express 13 (2005) 2678.
[3] T. Tanabe et al., CLEO/QELS2005,QDPA5, Baltimore (2005) .


Fig. 1. Electron microscope image of the silicon PhC nanocavity (top). Linear transmission spectrum (bottom).
Fig. 2. Recorded switching operation of the signal waveform as a function of the control pulse energy.

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